Yep, you read that right. The idea is to create an illusion which is normally not able to be perceived by a human, and then a second spell which then allows the wizard to perceive the special effect. Such as an illusion using ultrasonic sound or ultraviolet light and the caster has a second spell which detects them.
Why? Well I was thinking about how an Ars Magica Magus might create their own heads-up system, or mark allies before battle, or a way to tag people they meet as significant. This way their tags are not readily readable by everyone, and the target need not know. Alternatively the illusion might almost match the true image of the target, which is then highlighted by a special Intellego spell.
The complexity in building a HUD is effect duration and memory. Effect duration can be solved by investing the effect into an enchanted device, which then maintains concentration and can be enhanced with greater penetration. Memory cannot, and this is why the spell creates a marker on the target so they can see the detail.
As a stand alone spell, intended to be used over a longer time period.
Paint with Invisible Brushstrokes
Creo Imagonem / Muto 20, R: Voice, D: Moon, T: Individual
This spell creates a simple visual illusion using tailored species which are not normally perceivable by humans.
(Base 1 to create a visual illusion, +2 Voice, +3 Moon, +1 moves with the target, +1 for complexity)
Paint with Invisible Sigils
Creo Imagonem / Muto 20, R: Touch, D: Sun, T: Group
This spell creates a simple visual illusion upon all members of the group touched, using tailored species which are not normally perceivable by humans. The sigil is is a simple coloured marking, up to a pace in any direction. The intent is to mark allied companions and soldiers before battle which allows them to be easily differentiated, but it may also be used to mark any group.
(Base 1 to create a visual illusion, +1 Touch, +2 Sun, +1 moves with the target, +1 for complexity, +2 Group)
Then the spell to perceive the typically invisible illusions.
Perceive the Brushstrokes of Invisible Paint
Intellego Imagonem 15, R: Personal, D: Concentration, T: Vision
Allows the caster to perceive any normally invisible illusion or “brushstrokes” created with the associated spell.
(Base 1, +1 Concentration, +4 Vision, +1 for complexity)
Ref: A discussion thread on the topic in the official Ars Magica forums. This and other spells can be found in the Grimoire of New Spells for Ars Magica.